Lighting components which are based on semiconductor light sources such as, for example, LEDs provide a serious alternative to traditional fluorescent lamps, high-pressure discharge lamps or incandescent lamps. In principle, LEDs not only have a high conversion efficiency, a high optical efficiency, a long expected life and low operating costs, but also many other advantages. In some applications, an LED-based lighting system may include a power supply unit which provides an LED operating current for a plurality of light source modules, each in turn containing one or more LEDs. For example, a light source module can have a circuit board, for example a printed circuit or a printed circuit board (PCB), on which the one or more LEDs are mounted. Such circuit boards can be pushed into rails of a luminaire or plugged into female connectors on a main circuit board on which the power supply unit can be located.
In various applications or installations of an LED-based lighting system, the number of LEDs or light source modules required will be different in each case. For example, the number of LEDs or light source modules is to be matched to the required light emission of a specific installation. In general, the value of the LED operating current which is provided by a power supply unit is to be matched to the number of LEDs or light source modules to be supplied power by this power supply unit. If a single power supply unit is intended to be used in a multiplicity of LED-based lighting systems with different numbers of LEDs or light source modules, the power supply unit must contain an apparatus for setting the setpoint value of the LED operating current, which apparatus matches the operating current requirement to the different light source modules corresponding to the different number of light sources contained therein. At present, the number of LEDs and light source modules which are intended to be contained in a specific LED-based lighting system is fixed at the time of manufacture of this LED lighting system. If the same power supply unit is intended to be used in different LED lighting systems with a different number of light source modules, the power supply unit needs to be programmed for the intended LED lighting system at the time of manufacture, with the result that the LED operating current provided is appropriate for the specific number of light source modules which are contained in the intended LED lighting system.
As soon as a light source module with a relatively short life needs to be replaced during the relatively long life of an LED− based lighting system, the actual problem on which this invention is based arises: the advancement on the component part level of the LED is so serious at present that a light source module of the same type will emit much more light or will require substantially less current for the same emitted light if it is, for example, three years younger than the comparison module. Therefore, not only the specification present at the time of manufacture of the lighting system, but also the time per se play a significant role in this.
This problem has been addressed by setting up data interchange between the power supply unit and the light source module. Data interchange in this case means that the light source module transmits some information to the power supply unit, relating to the current requirement of the module for fulfilling its optical specification or its working temperature for the purpose of reducing the value of the current provided when a certain temperature limit value is exceeded. Various approaches are known for the interchange of this information between the light source module and the power supply unit. Buses can be used for data interchange. Known in this case are, for example, analog buses such as the 1 . . . 10 V interface or digital buses such as DALI (digital addressable lighting interface). Likewise known technologies are simple resistance networks, which can be measured by the power supply unit and transmit the current requirement of the light source module just connected or the light source modules just connected to said power supply unit. DE 100 51 528 A1 discloses an interface in which a special resistance, a so-called current-setting resistance, is connected between a third line and the negative supply line. If a plurality of light source modules are connected to a single power supply unit, the resistances are connected in series or parallel with one another, and in this way a summation signal is passed back to the power supply unit in order to define the total current requirement. The German patent application 102011087658.8 likewise discloses resistances for defining the current requirement of each individual light source module, i.e. module-specific current-setting resistances.
The bus solutions have the disadvantage of two additional connecting lines being required. The resistance solutions only require one additional connecting line, but the evaluation of the resistance network and the setting of the current value resulting from this can become very complicated.
Since complete lighting systems including a power supply unit and light source module(s) have appeared on the market, various companies have attempted to adopt a common approach for putting into operation the communication between the two component parts of the above systems; likewise, some digital protocols are in use for the more complicated high-end systems, but the latter technology is not the background of the present disclosure and needs to be discussed separately.
The company Osram, for example, has already proposed an interface which is also capable of providing an auxiliary power to an active circuit for thermal derating on a light source module. In this interface type, a current-setting resistance on the light source module in conjunction with a pullup resistance in the power supply unit forms a voltage divider, with the purpose of forming a center-point voltage which defines the output current of the power supply unit. An operational amplifier on the light source module begins to limit this center-point voltage and therefore the operating current provided as soon as the module overheats. The company Philips has proposed another interface in which one signal line is connected to the current-setting resistance and another signal line is connected to a temperature-sensitive resistance, and in which the thermal derating is performed by the power supply unit itself without any active component on a light source module being required.
Both of the last-mentioned interfaces require a third extra line for the common signal ground feedback and use a voltage generated by the current-setting resistance on the light source module for setting the setpoint operating current value in such a way that the operating current is set to be higher the higher the voltage across the current-setting resistance or across the current-setting resistances is.
Recently, the company Osram has proposed a slightly modified interface which is based on the 1 . . . 10 V bus mentioned already above, but modified by a precision current source in the power supply unit which makes it possible to achieve precise setpoint operating current value setting with only a single current-setting resistance per light source module. A further modification of this interface in turn consists in replacing the current-setting resistances on the light source modules with Zener diodes.
At present, a new demand is emerging on the market: the possibility of connecting various modules in parallel with one another and supplying power to said modules jointly by one and the same power supply unit. The operating current provided by this power supply unit in this case needs to correspond to the sum of the nominal current values of all of the light source modules connected thereto at that time, and the capability of thermal derating also needs to be maintained in the case of multi-module arrangements. A thermal derating signal on a data line should finally even be dominant with respect to a summation current setting signal.
Nevertheless, it is necessary to configure the lighting systems to be simpler, which at present results in the reduction in the number of the additional data lines. Bus-based interfaces require at least four lines, two for the light source module operating current and at least two for the bus.
New characteristics for meeting the requirements are envisaged:                a plurality of modules are intended to be connectable in parallel and capable of being supplied power by one and the same power supply unit using the same interface. In this case, the individual modules are considered to be the same as one another, or at least are considered to be modules which have the same operating voltage as one another.        the interface for setting the operating current should have a reduced number of lines and should be as simple as possible, for cost reasons, in particular on the side of the light source modules.        
All of the interfaces previously proposed and known are not capable of correctly supporting multiple connections of light source modules. The evaluation circuits for the interfaces are also complex in terms of manufacture. A novel interface is proposed which is simpler to manufacture.